The invention relates generally to semiconductor optical amplifiers and, more particularly, to lightwave systems and networks utilizing such amplifiers.
Optical amplifiers are commonly used in lightwave communication systems as in-line amplifiers for boosting signal levels to compensate for losses in a transmission path, as power amplifiers for increasing transmitter power, and as pre-amplifiers for boosting signal levels before receivers. In wavelength division multiplexed (WDM) systems, which combine many optical channels at different wavelengths for transmission as a composite signal in an optical fiber, optical amplifiers are particularly useful because of their ability to amplify all channels simultaneously.
Erbium-doped fiber amplifiers are predominantly used in current WDM communication systems because of their gain characteristics and ease of coupling with optical fiber. Erbium-doped fiber amplifiers are particularly desirable for intensity modulated digital optical communication systems, wherein the light intensity of signal channels is modulated to represent the xe2x80x9c1xe2x80x9ds and xe2x80x9c0xe2x80x9ds of digital data. In particular, slow gain dynamics allow erbium-doped fiber amplifiers to provide constant gain to all signal channels in a WDM system regardless of bit transitions in the intensity modulated bit patterns. However, despite their usefulness in long haul transmission applications, the disadvantages of erbium-doped fiber amplifiers are well known. For example, erbium-doped fiber amplifiers are expensive and, as a result, do not provide the most cost effective solution for applications such as metropolitan optical networking and the like. Moreover, erbium-doped fiber amplifiers have a relatively narrow usable gain bandwidth which will become more of a problem in emerging long haul systems which have higher channel counts and which will use new optical fiber having a wider usable bandwidth.
By contrast, semiconductor optical amplifiers are comparatively inexpensive, have a large gain bandwidth, and can be easily integrated with other devices. However, semiconductor optical amplifiers have several limitations which have limited their use in optical communication systems to date. In particular, the fast gain dynamics and nonlinear gain characteristics of semiconductor optical amplifiers can be problematic. For example, gain changes quickly as input power changes and is not constant for the modulation speed of current communication systems, thus resulting in problems such as inter-modal distortion and saturation induced crosstalk, i.e., cross-saturation.
Briefly, cross-saturation results when intensity modulation in one channel leads to modulation of the gain available for other channels. For example, the gain of a specific channel is saturated not only by its own power, but also by the power of the other channels in the system. Cross-saturation is particularly problematic in intensity modulated systems because the channel power changes with time depending on the bit pattern. The signal gain of one channel then changes from bit to bit, and the change depends on the bit patterns of the other channels. Such gain fluctuations can result in detection errors which degrade overall bit error rate performance. Cross-saturation can be avoided by operating in the small signal region, i.e., unsaturated region. However, this solution is not practical for WDM systems which traditionally operate in the saturation region because of pumping efficiencies and other system considerations, e.g., high saturated power needed for wide dynamic range and high signal to noise ratios.
For more information on nonlinear distortion effects in semiconductor optical amplifiers, see, e.g., Inoue, xe2x80x9cCrosstalk and Its Power Penalty in Multichannel Transmission due to Gain Saturation in a Semiconductor Laser Amplifierxe2x80x9d, Journal of Lightwave Technology, vol. 7, no. 7, July 1989; Saleh et al., xe2x80x9cEffects of Semiconductor-Optical-Amplifier Nonlinearity on the Performance of High-Speed Intensity-Modulation Lightwave Systemsxe2x80x9d, EEE Transactions on Communications, vol. 38, no. 6, June 1990; Simon et al., xe2x80x9cTravelling Wave Semiconductor Optical Amplifier with Reduced Nonlinear Distortionsxe2x80x9d, Electronics Letters, vol. 30, no. 1, January 1994; and Tiemeijer et al., xe2x80x9cReduced Intermodulation Distortion in 1300 nm Gain-Clamped MQW Layer Amplifiersxe2x80x9d, IEEE Photonics Technology Letters, vol. 7, no. 3, March 1995, all of which are herein incorporated by reference in their entirety.
To date, most attempts at solving the aforementioned problems have been limited to device-oriented solutions and have been predominantly directed towards problems for the small-signal model, i.e., transmissions in the small-signal gain region. Consequently, these approaches have not been particularly useful for WDM systems and the like.
Substantially error-free communications is achieved in an optical communication system comprising optical amplifiers according to the principles of the invention by detecting bits transmitted in the amplified optical signal according to a detection threshold that is derived as a function of a maximum power level associated with a first bit value, e.g., bit xe2x80x9c0xe2x80x9d, and a minimum power level associated with a second bit value, e.g., bit xe2x80x9c1xe2x80x9d. Importantly, this detection scheme can be used to accurately detect the bit patterns in the amplified signal even in the presence of nonlinear distortions such as inter-modal distortion and saturation induced crosstalk. In particular, accurate detection of individual bits in each of the channels is possible even with the nonlinear distribution of power in each of the channels resulting from gain variations caused by the nonlinear distortions.
According to one illustrative embodiment of the invention for use in a wavelength division multiplexed (WDM) system comprising semiconductor optical amplifiers, the detection threshold is set at a level corresponding to PTOTAL/2N, where PTOTAL represents the total power in the WDM signal and N represents the number of optical channels in the WDM signal.
According to another aspect of the invention, we have discovered that the effect of gain variations becomes smaller as the number of optical channels within the wavelength division multiplexed signal increases. In particular, a smoothing effect is realized for the total effective saturation power as gain variations decrease as a function of an increase in the number of channels. As such, the performance of a semiconductor optical amplifier according to the principles of the invention approaches the linear performance of fiber amplifiers as the number of channels increases.
A system operated according to the principles of the invention therefore includes all the benefits of semiconductor optical amplifiers, e.g., lower cost and large gain bandwidth, while avoiding the problems associated with nonlinear performance of semiconductor optical amplifiers and without requiring significant device changes as suggested by the prior art. Consequently, such a solution can be readily implemented and advantageously used, especially in metropolitan area optical networking applications where cost is a primary consideration.